The invention relates to a method for generating a primary binary signal having a predetermined spectral shape in a predetermined frequency range, in particular having a notch in the power spectrum in a predetermined frequency range. The invention further relates to a device for generating a primary binary signal having a predetermined spectral shape, to a binary signal having a predetermined spectral shape and to a record carrier for comprising such a binary signal.
A record carrier for optically detectable data storage and an apparatus for recording data on such a record carrier are disclosed in U.S. Pat. No. 5,682,365. The record carrier described therein is provided with periodic track variations, also called a xe2x80x9cwobblexe2x80x9d, whose period corresponds to a frequency for which a power spectrum of the digitally coded information substantially exhibits a zero point. When scanned by a beam of radiation, the periodic track variations produce periodic modulation in the reflected beam intensity of a frequency corresponding to the period of the track variations in order to generate a clock signal of a frequency equal to the bit frequency during recording or reproduction. This power spectrum of the primary binary signal, which is the binary signal storing the digitally coded information, thus has a sharp single-frequency notch, i. e. has a zero point at a certain frequency which frequency corresponds to the period of the periodic track variations, i. e. of the wobble signal. According to U.S. Pat. No. 5,682,365 a radial wobble is used in the servo tracks of a disc-shaped optical record carrier to provide a tracking signal.
The wobble signal can be considered as a secondary signal. In U.S. Pat. No. 5,682,365 a sinusoidal wobble is used. Other applications use a phase-modulated wobble to accommodate a secondary binary signal. A xe2x80x981xe2x80x99 channel bit corresponds in that case to a fixed number of wobble periods with positive amplitude (phase 0xc2x0), a xe2x80x980xe2x80x99 channel bit corresponds to the same number of wobble periods with a negative amplitude (phase 180xc2x0). The spectral shape of the channel bitstream of the secondary binary channel depends on the channel code used. In case the bi-phase code is used (with only two codewords, (+1, xe2x88x921) and (xe2x88x921, +1)), the spectrum is given by 2 sin2 xcfx89/2, with xcfx89 the frequency (xcfx89=xcfx80 at the Nyquist frequency). For the wobble channel, the spectrum of the secondary binary signal will be positioned centered at the wobble frequency. The extent of the spectrum of the secondary binary signal relative to the wobble frequency is determined e.g. by the chosen fixed number of wobble periods per channel bit.
A single-frequency notch is quite good in the central region around the single frequency of the pure wobble signal since an interference between the secondary binary signal and the primary binary signal at this single frequency is well suppressed. In general, the quality of the secondary signal accommodating a binary information stream, like the phase-modulated wobble signal, is not only determined by the interference on its central frequency, but by the interference over the whole frequency range that is spanned by the power spectral density of the secondary binary signal. This means that a single-frequency notch being quite good with respect to interference in the central frequency region of the PSD still leads to disturbing interference in the exterior regions of the PSD.
It is therefore an object of the invention to provide a method for generating a primary binary signal having a predetermined spectral shape such that interferences between the primary binary signal and the secondary binary signal are suppressed as much as possible in order to be able to detect the secondary binary signal free of disturbances caused by the primary binary signal. Further, it is an object of the invention to provide a corresponding device for generating a primary binary signal, a binary signal having a predetermined spectral shape and a record carrier for comprising such a binary signal.
This object is achieved by a method for generating a primary binary signal as claimed in claim 1. According to this method the primary binary signal comprises a modulation step from data-words into channel-words whereby in general a limited degree of freedom in the choice of the modulation of the data-words is available. This freedom of choice is according to the invention used to generate a primary binary signal having a predetermined spectral shape, in particular having a spectral notch, which is determined via a spectral weight function the shape of which is tailored to the spectral extent of a secondary binary signal such that the secondary binary signal can be accommodated to the spectral shape of the primary binary signal. In particular, the secondary binary signal can be accommodated spectrally at the position of the notch in the power spectrum of the primary binary signal and the width of the notch is determined by the spectral extent of the secondary binary signal. In other words, the shape of the weight function used to determine the spectral shape of the primary binary signal is selected such that the spectral shape of the primary binary signal is adapted to the spectral shape of the secondary binary signal to avoid crosstalk between these two binary signals and to enable read-out and decoding of the secondary binary signal without disturbencies caused by the primary binary signal.
A known single-frequency notch at zero frequency may, however, be additionally employed. This means that the wide-notch, preferably provided according to the invention, can be provided on top of a single-frequency notch around DC. Such a notch around DC is needed in view of DC-control, i.e. for a separation of the information signal from low-frequency disk noise, necessary for control of the slicer level, and in order to avoid interference of the information signal with the servo systems.
A preferred embodiment of the method comprises the steps as claimed in claim 2. According to this embodiment a sum value computed on the basis of candidate channel bitstreams of the primary binary signal is used as a criterion for the determination of the channel-words forming the primary binary signal. Depending on the actual use of the primary binary signal the content of the data-words which are modulated into the channel-words can be selected completely free or has to follow certain restrictions which limit the selection of the channel-words in step a) of this embodiment. Different channel-words are then selected and the sum value for these channel-words is then determined, preferably by a bit-by-bit recursive calculation of a convolution-type of sum, to find the lowest sum value. The channel-word resulting in the lowest sum value is then selected for this given data-word location. These steps are then repeated for several or all data-word locations of the primary binary signal to find the channel-words resulting in the lowest sum values for the respective data-word locations. By use of the weight function in the step of determining the sum value the spectral shape of the primary binary signal can be influenced as intended. The aimed spectral weight function determines the values of the tap-coefficients that are to be used in the evaluation of the convolution-type of sum.
In a further embodiment the set of possible channel-words at a given data-word location is generated by the encoding freedom of the modulation code used either in the choice of merging bits, (EFM-modulation for CD) or in the use of substitution tables (EFMPlus-modulation for DVD) or in the use of extra control bits (17PP, Parity-Preserve modulation of the rewritable format of DVR) which are used in the channel modulation. In general, the encoding freedom of a modulation code used depends on the application of the primary binary signal, i. e. in certain applications the content of data symbols at a given data-word location is completely free whereas in other applications the content of data symbols is fixed. In an application where the content of the data symbols is completely free the selection of the channel-word in step a) could also be performed by a random selection of the value of the data symbol. The set of possible channel-words at a given data-word location can thus also be generated by variation of the data-words.
In a preferred embodiment of the invention the weight function is a Gaussian function. Such a weight function is preferably used if the power spectrum of the secondary binary signal has a shape that can satisfactorily be approximated by a Gaussian function. The parameters of the Gaussian function can be determined according to the spectral extent of the secondary binary signal.
Another embodiment of the invention as claimed in claim 8 provides a method of determining the sum value by bit-by-bit recursive calculation. This recursive calculation is quite a simple method of determining the criterion, which is the sum value, for the selection of the channel-word at a given data-word location. This calculation is performed bit-wise which means that the sum value is calculated bit-by-bit for all bit positions in the channel-word, e. g. if the channel-word has 17 bits (as in the case for the EFM code, with words of 14 bits concatenated by 3 merging bits) this step is performed 17 times. In the tap-coefficients of the convolution-filter used therein the parameters of the weight function, in this case of the Gaussian function, are included to determine the spectral shape of the primary binary signal.
According to further embodiments of the invention as claimed in claims 9 and 10 the primary binary signal is used in the servo-tracks or in the lead-in area of an optical record carrier like a CD or DVD.
In a preferred embodiment the primary binary signal is used in the lead-in area of an optical record carrier where the secondary binary signal is a wobble signal realized by wobbling the information track and stored in a wobble channel. According to this embodiment the weight function is determined such that the power spectrum of the wobble signal fits in the spectral notch generated in the primary binary signal and the wobble channel is used for storing data in the lead-in area of the record carrier. Preferablyxe2x80x94as claimed in claim 11xe2x80x94the wobble-channel is used as a side-channel that contains a wobble key for decryption of data stored in the data area of the record carrier, preferably in a Read Only Memory disc (ROM). Such a wobble key cannot be copied since the wobble channel is created in the mastering step of a ROM disc. A recordable disc or a rewritable disc can also have a wobble, but that is in view of address information and/or clock generation for the write-process. Such a wobble is thus always different from the wobble in the ROM disc. The wobble in the ROM disc contains information that is coupled to the content of the disc, so that the wobble key is needed for decryption.
A significant part of commercially available CD-audio drives use a 3-beam set-up for radial tracking instead of the Radial Push-Pull method. The Radial Push-Pull method yields a qualitatively good wobble signal because of the subtraction of the signals detected on the two detector halves, aligned in the direction normal to the track direction (radial direction) in the exit pupil of the objective lens. The subtraction operation effectively cancels uniform noise contributions that are present on both detector halves. In contrast the Central-Aperture (CA) signal of one of the side-spots of the 3-beam set-up is of much worse quality in general. Part of the noise is due to the interference with the EFM-data of the primary binary signal. An EFM-pattern can have a long-range periodicity with a period close to that of the wobble period which is in one practical application 196 EFM-bits. In such case it is difficult to distinguish a contribution of the wobble-effect from a long-range code-effect that can occur with some probability. In the spectrum the interference of signals can directly be seen if the signals have significant content at the same spectral frequency range. Therefore significant content of the EFM-code in the spectral range of the wobble is a noise source for the secondary binary signal accommodated in the wobble channel, and due to the 3-beam side spot detection, there is already a large portion of low-frequency noise present. The method according to the invention, however, prevents EFM disturbence on top of the wobble signal and the normal noise when used for generating the primary binary signal in the lead-in area of an optical record carrier, wherein the secondary binary signal is a wobble signal.
Further embodiments of the inventions may be found in further dependent claims.
The invention is also embodied in a device for generating a primary binary signal as claimed in claim 14, in a binary signal as claimed in claim 15 and a record carrier for storing a binary signal as claimed in claim 16. Preferably the record carrier is an optical record carrier like a CD or DVD, especially a ROM-type disc, having a lead-in area comprising a wobble channel used for storing a wobble key. It shall be understood, that these devices, the method for decoding, the binary signal and the record carrier can be further developed and that there are further embodiments thereof, which further developments and further embodiments are identical or similar to those described above with reference to the method and laid down in the subclaims dependent on claim 1.